Electronic system modules and method of fabrication

ABSTRACT

This specification describes techniques for manufacturing an electronic system module. The module includes flexible multi-layer interconnection circuits with trace widths as narrow as 5 microns or less. A glass panel manufacturing facility, similar to those employed for making liquid crystal display, LCD, panels is preferably used to fabricate the interconnection circuits. A multi-layer interconnection circuit is fabricated on the glass panel using a release layer. A special assembly layer is formed over the interconnection circuit comprising a thick dielectric layer with openings formed at input/output (I/O) pad locations. Solder paste is deposited in the openings using a squeegee to form wells filled with solder. IC chips are provided with gold stud bumps at I/O pad locations, and these bumps are inserted in the wells to form flip chip connections. The IC chips are tested and reworked. The same bump/well connections can be used to attach fine-pitch cables. Module packaging layers are provided for hermetic sealing and for electromagnetic shielding. A blade server or supercomputer embodiment is also described.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of co-pending application Ser. No.10/702,235 filed Nov. 5, 2003, which is a continuation-in-part ofco-pending application Ser. No. 10/237,640 filed Sep. 6, 2002 whichclaims priority to Provisional Application Ser. No. 60/318,271 filedSep. 7, 2001.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK.

Not applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to connections between components, andmore particularly to connection arrays where each connection in thearray includes a bump inserted into a well containing bonding material.

2. Description of the Related Art

The number of input/output (I/O) connections required by integratedcircuit (IC) chips is increasing, to several hundred for recentmicroprocessor chips. As verification of complex designs becomes anincreasing portion of the total design activity, it is desirable toincrease the I/O count further, to provide access to more internal nodesfor testing. Flip chip assembly methods have helped to provide more I/Oconnections because they provide an area array of connections across theentire face of an IC chip, rather than just at the perimeter as withwire bonding. However, even with flip chip assembly it continues to bedesirable to reduce the pad pitch, the distance between bonding padcenters, in order to achieve more I/O connections per unit area of ICchip.

A recent advance in flip chip assembly capability has been theintroduction of stud bumping machines that can provide gold stud bumpson IC chips with pad pitches of less than 100 microns. However, to takeadvantage of this capability, the system board or package that receivesthe bumped devices must have fine traces in order to route all of thesignals in the space available with not too many layers, plus it mustsupport bonding pad pitches less than 100 microns. The most recentpackaging technology to be commercially introduced is called land gridarray, LGA. It builds up the wiring layers by plating a base layer ofcopper that has been patterned with photo resist. The external terminalpitch claimed for this packaging method is “less than 0.5 mm”. This isnot sufficient for direct bonding of IC chips; in fact none of theavailable printed circuit board technologies can support direct mountingof bumped devices at a pitch of 100 microns or less. The currentinvention is capable of pad pitches of less than 100 microns withoutusing any redistribution wiring layers; it also includes a viable methodfor reworking defective IC chips at this bonding density.

Indium-based solders have been developed that form reliable bonds withgold structures such as stud bumps. An example is Indalloy 290 availablefrom Indium Corporation of America, Utica, N.Y. This alloy contains 97%indium and 3% silver and melts at 143° C. Since the solubility of goldin this solder is very low, brittle inter-metallic compounds are not asignificant problem, and the integrity of a gold stud bump can bemaintained through multiple rework cycles.

For many years the minimum trace width available from printed circuitboard vendors has been around 100 microns using glass epoxy laminates.Recently, Unitive, Inc. of Research Triangle Park, N.C., USA, hasproduced interconnection circuits using a spin-on dielectric called BCB(benzocyclobutene) having a copper trace width of 12 microns and a spacebetween traces of 13 microns. The current invention preferably usesaluminum conductors and BCB dielectric and is capable of achieving tracewidths of 5 microns or less, together with a trace pitch of 10 micronsor less. These fine aluminum traces are well suited for low power andlow frequency applications. Higher current applications may use thickerand wider traces, and may substitute copper for aluminum. High frequencyapplications can employ the methods described herein if additionaleffort is applied to form traces with controlled impedance; for example,if differential pairs and ground reference planes are employed thefrequency range may be extended to the order of I GHz, depending ontrace lengths and other specifics of the application.

One way to achieve fine line interconnection circuits is to employ asemiconductor fabrication facility and to build the interconnectioncircuit on a silicon wafer; hence the term, wafer level packaging, WLP.The precision of the associated photolithographic methods, the cleanroom environment with low particulate count, and the advanced substratehandling equipment of such a facility can all contribute to high-densityinterconnection circuits. However, the application of IC chipmanufacturing facilities to this problem is more than what is required.An intermediate alternative is to apply the manufacturing resources of aglass panel fabrication facility, where the minimum feature sizes are 10to 20 times larger than for IC chips (but still adequate for the mostadvanced assembly processes), and the manufacturing cost per unit areais less than 5% of the cost per unit area of IC chips. In addition, theglass panel fabrication facility can produce system boards of any sizeup to around two meters on a side for the latest panel fabricationfacilities, whereas the largest wafers produced have a diameter of 300mm.

In order to avoid the rigidity and weight of the glass substrate, and toprovide better thermal access to the heat producing components forcooling them, it is usually preferable to discard the glass carrierafter most of the processing is done.

Typically, the fine trace capability of WLP has been used to createredistribution circuits that map from the fine pitch available with flipchip bonding to the coarser pitch of a printed circuit board. Thecurrent invention eliminates the redistribution circuits because theprinted circuits produced (termed interconnection circuits) include finefeatures that easily accommodate the fine pitch of the flip chipbonding.

Power supply voltages and signal voltage swings are reducing with eachnew generation of IC chip technology. To achieve the necessary, noisemargins during testing it is generally necessary to connect test pointsto test circuits using short leads. This is typically achieved byemploying a test head that is located close to the test points. The testhead provides a set of pin electronics for each signal tested; the pinelectronics typically include high-speed sampling circuits andcomparator circuits, along with matrix switches and relays to map testpoints to test pins. Providing test chips on the motherboard (systemboard) is another way to achieve high speed functional testing. The testchip or chips can be placed close to system buses for sampling signalactivity at high speed, and this test method is recommended forelectronic modules of the current invention. Said test chips incorporatehigh speed sampling circuits and comparators; they work in concert witha test support computer that is cabled to the system and communicateswith the test chips and the system at relatively low data rates. Moreaccurate and complete testing of components is provided when they aretested in their real system environment rather than being tested asindividual components using test vectors that represent a simulation ofthe system environment. The system environment is preferably createdwith the actual system running a test version of the system'sapplication code, programmed in the language of the application ratherthan a special test language. This way, the system architects can alsobe the test architects. This can lead to improvements in testdevelopment time, test effectiveness and cost, compared with theconventional approach that includes a test program in a specialized testlanguage, simulated test vectors, and a general-purpose tester. Thistest method provides focus on the system level requirements, as opposedto component level requirements. If the system level requirements aresatisfactorily met, then the minutiae of component level characteristicsbecome irrelevant. Alternatively stated, only the functions relevant toproper system function are tested; this is a much more manageable set ofrequirements than the total set of functions that all the assembledcomponents are capable of performing.

If necessary, multiple IC chips may be employed to test the entire rangeof digital, analog, and RF functions of a particular product. Addingthese chips to the system using the current invention is not asexpensive as in the past because the test chips will be manufactured involume and will have low unit costs, and the packaging and assembly costwill be minimal, as will be further discussed. In summary, a mini-testermay be included with every module produced, but the cost of this testermay be well justified by the system level assembly and performancebenefits, and the reduction in system development time.

Module verification can be performed at an elevated temperature byheating the glass carrier underneath the module (circuit assembly orassembled system board). By providing a pre-determined test temperatureto the entire circuit assembly, a speed grade can be associated with themodule, as has been done in the past at the component level. Greateremphasis can be placed on environmental stress testing at the modulelevel. Accelerated life testing can also be performed early in the lifecycle of a product, and lessons learned about particular components canbe incorporated into the module level test.

R. K. Traeger, “Hermeticity of Polymeric Lid Sealants”, Proc. 25^(th)Electronics Components Conti, 1976, p. 361, has documented the waterpermeabilities of silicones, epoxies, fluorocarbons, glasses and metals.Traeger's data shows that, in terms of providing a barrier to water, alayer of metal that is 1 micron thick is approximately equivalent to alayer of glass that is I mm thick, and also equivalent to a layer ofepoxy that is 100 mm thick. Hermetic packaging techniques andelectromagnetic shielding techniques can be applied at the module levelto improve both performance and manufacturing cost. The currentinvention describes a method for fabricating a metal envelope thatencloses almost the entire module, substantially attenuating theinterference from individual components and the wiring between them.Cost can be reduced because hermeticity and shielding are provided witha simple process applied once to the entire system, rather than beingaddressed individually at each of the components.

Such a complete module or system fabrication process can be achieved ifa panel fabrication facility is applied to the complete set of modulemanufacturing steps, including high-density cables and connectors andback-end processing for hermeticity and shielding. Sub-processes for thefollowing structural elements are included: multi-layer interconnectioncircuits; a special assembly layer that may be required for directattachment of IC chips (wells filled with solder at each I/O pad);module access ports (arrays of test points and system interconnectshaving wells filled with solder at each I/O pad); module access cablesor test fixtures for connecting between the module access ports andexternal systems; and the back end module layers that provide bothhermeticity and electromagnetic shielding.

Usually structures for direct chip attach require an epoxy under-layerbetween direct mounted IC chips (flip chips) and the package or circuitboard. The purpose of the under-layer is to provide mechanical strengthto withstand repeated thermal cycling without developing cracks in thearea of the flip chip bonds. The thermal stress typically arises becauseof differences in coefficients of thermal expansion (CTEs) between theIC chip material and the board material. Gelatinized solvents have beenused to dissolve the epoxy during rework of defective chips; theytypically leave a residue that must be cleaned off. The process ofcleaning off the epoxy and the residue has often resulted in damage tothe fine pitch bonding leads, to the point where they cannot be reliablyre-bonded. This under-layer is unnecessary with the current inventionbecause mechanical strength is provided at each bond by the physicalstructure of a stud bump mated with a well filled with solder. Also thefinal interconnection circuit is flexible so that thermally inducedstresses are substantially eliminated in the region of the flip chipbonds. Without the thermally induced stress, no cracking will occur.Thermal stresses are still present during assembly (because thecarrier/interconnection circuit is rigid at this point), but are avoidedduring operation in the field (when the carrier is removed and theinterconnection circuit is flexible). The number and extent of thermalcycles endured during assembly are more predictable and controllablethan thermal cycles arising from operation in the field. Stress testingin the laboratory can be used to quantify the acceptable temperaturelimits, and assure crack-free circuit assemblies. However, using theproposed flip chip assembly method, it may be necessary to limit themaximum size of IC chips assembled, to limit the maximum strain inducedby thermal mismatch during assembly.

Replacement of defective chips (rework) is much easier if there is noepoxy under layer to be removed. Also, in the preferred embodiment thereare no delicate traces to be damaged during rework because each I/O padis provided with a well filled with solder, as will be furtherdescribed. Attaching IC chips onto flexible substrates is referred to inthe art as “compliant packaging”.

BRIEF SUMMARY OF THE INVENTION

A glass substrate for 5^(th) generation fabrication of LCD circuits istypically 1100 by 1250 mm in area, and 0.7 mm or 1.1 mm thick, which canbe used in carrying out the present invention. However, the glasscarrier of the current invention can be of any size. The unitmanufacturing costs of interconnection circuits and related circuitstructures of the current invention are lower if larger glass panels areused. The glass or other rigid carrier provides mechanical support forall of the fabrication, component assembly, cable assembly, test, andrework process steps, and also has excellent dimensional stability. Thisdimensional stability transfers to the multi-layer interconnectioncircuits that are built up as a series of films on top of the glass.This transferred dimensional stability is a primary reason that fineline features such as trace width and space of 5 microns are possiblewith the current invention. It is also important however, that the finalversion of the interconnection circuit be flexible, because thisflexibility allows the use of an epoxy under layer to be avoided,leading to more robust rework processes for removing and replacingdefective chips. The flexibility also allows system boards to be foldedin compact devices such as cellular phones. Folding requires that thechips be arranged in rows, and the clear area between rows becomes apotential folding line.

A release agent is applied to a glass panel substrate, except for aclear region near the edges. The clear region is characterized by highadhesion between the glass and the polymer base layer to be subsequentlyformed on the glass. The high adhesion region provides an anchor thatfirmly attaches the polymer to the glass around the perimeter of thepanel. The release layer creates low adhesion between glass and polymer,so that after a circuit assembly has been built on top, it can bereadily peeled off.

Alternate layers of metal interconnect and dielectric such as aphoto-definable polymer are built up on the base layer. Two-levelcontacts are formed between adjacent metal layers, and stacked contactsare provided between groups of adjacent layers. Preferably eachinput/output (110) pad of an assembled IC chip is a tested node of themulti-layer interconnection circuit. At the center of each I/O pad astacked contact is preferably created, with stubs at every metal layer,for convenient routing of traces. The base polymer layer, the dielectriclayers and the metal layers are flexible, and when the multi-layerinterconnection circuit is subsequently removed from the glass panel, ittoo is flexible. Attached IC chips are usually not flexible (unlessthinned down by lapping), but if a folding line on the interconnectioncircuit is kept clear of IC chips, then the circuit assembly can befolded at the folding line.

While the multi-layer interconnection circuit is still attached to theglass carrier and before it is divided (singulated) into individualcircuits, it may be convenient to form a special flip chip assemblylayer that includes wells filled with solder at each I/O pad. Each wellis designed to accept a stud bump of an attached component. Preferably,gold stud bumps are formed at I/O pads of all IC chips to be assembled.One method of creating the wells employs a thick layer of polymer thatis applied on top of the interconnection circuit. Openings in this layer(wells) are formed at each I/O pad (bonding site). The cured polymerlayer forms a mask with openings. A squeegee is used to wipe solderpaste over the mask to fill the openings, thus forming a well filledwith solder at each of the I/O pads. An alternative method for creatingthe wells is to take advantage of the surface depressions that areassociated with stacked contacts, to provide a stacked contact at eachI/O pad, and to fill the wells so formed with solder paste using asqueegee.

The glass carrier may be diced with a diamond saw to separate individualcircuit assemblies from one another, providing a more convenient formfor component assembly, cable assembly, test, and rework. Theinterconnection circuit must itself be tested, before any assembly isdone. This test is preferably performed using a test fixture thatconnects through a module access port to an external tester. The moduleaccess port may include I/O pads (module access pads) for every node ofthe multi-layer interconnection circuit. The assembly and rework stepsrequire that IC chips and other surface mounted components are preciselylocated in order that the flip chip placement tool can accurately alignbonding sites on the components with corresponding bonding sites on theinterconnection circuit. Accordingly, the circuit assembly remainsattached to the glass carrier, and its dimensional accuracy ismaintained until these steps are completed.

A module cable with high-density interconnections is preferably attachedto each circuit assembly while the attached carrier provides dimensionalstability. Each circuit assembly is then separated from its glasscarrier by peeling the base substrate away from the carrier. A lowadhesion force is provided in these regions so that separation can beaccomplished without damaging the interconnection circuits or theattached components.

The methods for fabricating interconnections and bumps and wells can beapplied to cables as well as to circuit boards, as will be described.

Module level coatings including dielectric and conductive layers areapplied to the tested circuit assembly to create a circuit module thatis hermetically sealed and electro-magnetically shielded. To effectivelycool the dense circuit module, it may be bonded to a heat sink, and theheat sink maybe cooled with a circulating fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1C shows a corner fragment of a glass carrier in plan view,describing the process steps for creating a polymer base layer on arelease layer;

FIG. 1D is a cross-sectional view of section DD of FIG. 1C, and showsthe base polymer layer in relation to the release layer;

FIG. 2 is a plan view layout of multi-layer interconnection circuitsarrayed on a glass carrier;

FIG. 3A-3E shows a series of cross-sectional views depicting the processsteps for creating the first few layers of an interconnection circuit;

FIG. 4 is a cross-sectional view of a multi-layer interconnectioncircuit of the current invention;

FIG. 5A is a cross-sectional view of a stacked contact;

FIG. 5B is a plan view of a stacked contact, excluding I/O pad metal;

FIG. 6 is a plan view of an array of orthogonal metal traces with tracepitch, p; FIG. 7 is a schematic plan view of a circuit node connectingbetween I/O pads on separate IC chips;

FIG. 8A is a plan view of a circuit assembly of the current invention;

FIG. 8B is a cross-sectional view of section XX of FIG. 8A, andrepresents a circuit module in process;

FIG. 9A-9C shows structural cross-sections depicting a first method offlip chip assembly;

FIG. 10A-10D shows structural cross-sections depicting a second methodof flip chip assembly employing a special assembly layer;

FIG. 11A is a plan view of a fragment of an interconnection circuitafter additional processing to create the module access port;

FIG. 11B shows a test fixture of the current invention, in relation to acircuit assembly;

FIG. 12 is a cross-sectional view of a circuit assembly showing thefirst module level coatings;

FIG. 13A-13C shows structural cross-sections that illustrate a preferredmethod for connecting a module cable or a test fixture to a circuitmodule;

FIG. 14 is a schematic cross-sectional view of an RF sputtering machine;

FIG. 15 is a plan view of a module cable of the current invention,connected to a circuit assembly;

FIG. 16 shows a scribe mark on the glass carrier of a module cable;

FIG. 17A-17C is a series of cross-sections depicting the process stepsfor connecting a module cable to a circuit assembly;

FIG. 18A shows a sputtering chamber arrangement for coating the secondtopside module-level metal layer;

FIG. 18B shows a sputtering chamber arrangement for coating the bottomside module-level metal layer;

FIG. 19 is a cross-sectional view of a system module with attachedmodule cable of the current invention;

FIG. 20A is a top view of a cable of the present invention with an inputport and an output port.

FIG. 20B shows a cable having a port with a redistributed array ofinput/output pads.

FIG. 20C shows a cable with multiple fingers and multiple ports.

FIG. 21 is a flow chart summary of the process steps to create a testedcircuit assembly;

FIG. 22 is a flow chart summary of the additional steps to convert atested circuit assembly into a completed circuit module;

FIG. 23 is a plan view of a blade server component of the currentinvention; and

FIG. 24 is a cross-sectional view of an edge fragment of a circuitmodule attached to a heat sink.

It should be understood that for diagrammatic purposes some parts of thefigures are drawn to scale and some are not. For example, the verticaldimension of a layer that is not the focus of a particular drawing maybe contracted. In other figures the thickness of very thin layers isexpanded in order for them to show in the figure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a corner fragment of a glass carrier 1. Photo resist hasbeen patterned on the carrier, so that a border of resist, 2, having awidth of approximately 20 mm, surrounds the perimeter. FIG. 1B showsthat a film of release layer, 3, has been applied over the entiresurface of the glass carrier. A suitable material for the release layeris a fluorinated silicone such as F065 manufactured by Gelest, Inc., inMorrisville Pa., USA. This material is a single part gel. It can beapplied as a fog or fine spray, or using the spin-on method. A suitablethickness is 2-5 microns after curing, with 2 microns preferred. Atypical curing cycle is 125° C. for 25 minutes. This release materialhas a silane component that bonds well to glass surfaces, yet presents afluid-like interface to polymeric materials like the base layer to besubsequently applied. FIG. 1C shows the result of lifting the resist topattern the release layer, using a developer or resist stripper to swellthe resist; border 4 is clear of release material. A base layer 5 isthen applied. Base layer 5 is preferably a polymer, and is applied inliquid form with a metering roll, an extrusion coater, or using aspin-on or spraying method. A suitable polymer is Cyclotene, a polyimidemanufactured by Dow Chemical, also known as BCB (benzocyclobutene). Noadhesion promoters are used. After curing, a suitable thickness of baselayer 5 is 40-80 microns, with 50 microns preferred. FIG. 1D is across-sectional view of section DD of FIG. 1C, and shows the relationbetween glass carrier 1, release layer 3, and base layer, 5. Surfaceregion 6 is characterized by high adhesion between the base layer andthe carrier, and surface region 7 is characterized by low adhesionbetween the base layer and the release layer. Base layer 5 may later bepeeled off of surface region 7 without damage to the base layer or tocircuits built thereon. An alternative method for providing the baselayer to serve as the substrate for the interconnection circuits is tolaminate a preformed sheet of polymeric material onto the glass carrier,providing a combination of adhesives to effect strong adhesion insurface region 6 for anchoring the edges of the sheet, and weak adhesionin surface region 7 for facilitating later release.

FIG. 2 shows a glass carrier 1 with a width, W, of 1250 mm, typical of5^(th) generation LCD panels. A recent facility has been announced thatwill handle substrates 1870 mm×2200 mm. Although any size glass carriercan be used to carry out the present invention, larger sizes result inlower costs for the associated interconnection circuits, includingspecial flip chip assembly layers that may be required. Materials otherthan glass can be used for the carrier, providing they are rigid anddimensionally stable. Glass is preferred in the current inventionbecause it is well characterized as a substrate material, and it is usedwith mature panel manufacturing methods such as for LCD fabrication.Individual multi-layer interconnection circuits 20 are arrayed acrossthe glass carrier. In this example, interconnection circuits 20 measure4 inches by 2 inches, and 220 copies are arrayed on the carrier.Multi-layer interconnection circuits 21 and 22 have different sizesrepresenting other circuits to be manufactured. Border region 23corresponds to surface region 6 of FIG. 1, and is a region of highadhesion between base layer 5 and glass carrier 1. When glass carrier 1is diced into interconnection circuits 20, 21, and 22, it can be seenthat the resulting circuits include only regions of low adhesion, foreasily peeling away the individual glass carriers of interconnectioncircuits 20, 21, and 22. An alternative version of the layoutrepresented in FIG. 2 includes streets of high adhesion provided betweeneach of the interconnection circuits, included for improved dimensionalstability of the individual circuits such as 20, and these streets areremoved during the dicing operation.

FIG. 3A-FIG. 3E represents the preferred method for forming thin filmlayers of multi-layer interconnection circuit 20 on base layer 5. Thepreferred method and materials contained herein are better suited to lowpower electronic modules such as cell phones than high performancecircuits that may require copper conductors at greater trace thickness.The aluminum conductors may be arranged as transmission lines for highfrequency operation, requiring controlled horizontal spacing betweentraces as well as controlled vertical spacing between traces andreference planes (not shown), as is known in the art. The metal layersof multi-layer circuit 20 are preferably aluminum, deposited bysputtering in a vacuum chamber. A suitable thickness range is 1-2microns, with 1 micron preferred. The metal layers are masked usingconventional photolithographic methods known in the art, and arepreferably dry etched using plasma etching processes, also known in theart. The patterning of each layer typically includes coating with photoresist, exposing with light through a mask or reticule, developing theresist to form openings where the material is to be removed, and etchingof the layer through the openings in the resist. Alternative metals maybe used. In FIG. 3A a patterned trace of metal 31 is shown on base layer5. For example, this trace has a width of 5 microns and a thickness of 1micron. In between each layer of patterned metal is a layer of patterneddielectric, to provide isolation in the vertical direction between themetal traces in FIG. 3B, the substrate and metal traces have been coatedwith a planarizing layer 32 of a photo-definable polymer as theinter-layer dielectric. A suitable photo-definable polymer is photo BCB,benzocyclobutene, a photosensitive form of Cyclotene. The term“photo-polymer” shall be used hereinafter for this material. Thephoto-polymer is applied in liquid form to planarize the surface. Asuitable thickness of photo-polymer layer 32 is 2-4 microns aftercuring, with 2 microns preferred. In FIG. 3B a masked region ofphoto-polymer is exposed to light where the material is to remain. Theeffect of light on the photo-polymer is to form cross-linked moleculesthat become solidified (polymerized) and are not dissolved in thesubsequent development step. Polymer material that is not exposed tolight does not develop cross-links and is removed by the developmentstep. This is the same mechanism that occurs when patterning a negativephoto-resist, and is referred to as negative image development. FIG. 3Cshows the result of developing the polymer. The masked illumination ofthe exposure process causes photons to penetrate the surface of thepolymer. The photons spread laterally as they penetrate to lower depths,causing cross-linking as they spread. The net result is a patternedpolymer layer having tapered contact windows with an angle, B, ofapproximately 45°, as shown in FIG. 3C. Typically, contact windows havea curved profile that is approximated by the straight line shown. FIG.3D shows the result of depositing and patterning the next layer ofmetal, 34, to form the two-layer contact, 33 between traces 31 and 34.Because the contact window is tapered rather than vertical, and becausesputtering from a large area target provides multiple deposition angles,good metal coverage is achieved at the contact walls. FIG. 3E shows theaddition of the next photo-polymer layer 35, and patterning of the nextmetal layer as metal trace 3& The foregoing description teaches theformation of multi-layer interconnection circuits, with alternatingpatterned metal and patterned polymer layers, by repeating the foregoingsteps.

FIG. 4 is a cross-sectional view of an example multi-layerinterconnection circuit 20 of the present invention, introduced in FIG.2. Base polymer layer 5 is shown. Conductive trace 41 of first layermetal is shown, with width, w, of 5 microns or less, spacing, s, of 5microns or less, and thickness, t, of approximately one micron in thepreferred embodiment. It maybe desirable to arrange conductors onalternate layers to be generally orthogonal as in the figure, as iscommon practice for layout efficiency. A planarizing layer ofphoto-polymer, 42, has been applied over the first layer metal patternwith a preferred thickness of two microns after curing. A trace ofsecond layer metal, 43, forms a two-level contact with a trace 44 offirst layer metal. The next photo-polymer layer 45 again has a preferredthickness of 2 microns, and covers second layer metal 43 with athickness of 1 micron. Photo-polymer layer 45 provides a planar surfacefor deposition and patterning of a third layer metal such as trace 46.Trace 47 is fourth layer metal and connects using a two-level contact toa trace 48 of third layer metal as shown. Additional layers are built upin the same manner, as required, and in principle any number of metallayers can be provided. In the figure, trace 49 is on the eighth metallayer.

FIG. 5A shows an expanded cross-sectional view of a stacked contact 50at an input/output (I/O) pad. Metal traces on alternate layers arepreferably orthogonal as shown. A trace 51 of first layer metal is shownwith a suitable trace width of 8-12 microns in this contact structure,with 10 microns preferred. Planarizing layers of photo-polymer such as52 and 53 are used between each metal layer, as described in referenceto FIG. 3 and FIG. 4. Trace 54 of second layer metal contacts trace 51as shown. A contact stack of all metal layers is built up layer bylayer, with stubs provided for connecting metal traces at any level.Stubs are short metal traces that are provided to establish points ofaccess at each metal layer. Most of them will never connect to anythingelse. However, some of them will be extended into circuit traces of theinterconnection circuit. Trace stubs 54, 55, 56, and 57, are on metallayers 2, 4, 6, and 8, respectively. Similarly, at 90 degrees rotationfrom these stubs, the odd numbered metal layers also have similar stubs(not shown). Finally I/O pad 58 connects with the contact stack asshown. It may be convenient to build a stacked contact like 50 at all ofthe I/O pads. The width of I/O pad metal is approximately 85 microns inthe preferred embodiment, providing ample space for such a stackedcontact. It is convenient for the circuit board layout designer to knowthat every node in the circuit is available at all metal layers (usingthe stubs), and at a known location. FIG. 5B shows a reduced plan viewof the stacked contact of FIG. 5A, excluding I/O pad 58 to reveal thelocations of the stubs. The location of stubs on even-numbered metallayers is shown 59, and odd-numbered metal layers 60. The common area 61of the stacked contact is also shown.

Stacked contact 50 is necessarily larger in horizontal area thantwo-level contact 33, because of the long sloping contact walls. Thetrace pitch for parallel runs of metal is a critical parameter fordensely packed interconnection circuits. Parallel runs of metalgenerally require contacts to traces on other layers for effective tracerouting of a multi-layer interconnection circuit. To achieve minimumtrace pitch for such parallel runs, it is desirable to use contacts ofminimum size. This can be accomplished if contacts that are formed atlocations other than at the I/O pads are limited to two-level contacts.This is shown in FIG. 6. Horizontal traces such as 64 are on aneven-numbered metal layer. Vertical traces such as 65 are on an adjacentodd-numbered metal layer. Contact windows 66 and 67 are for two-levelcontacts; they are closely spaced but staggered, and have a minimumcontact area. The trace pitch p is consistent at 10 microns or less inboth directions, and not increased for traces with contacts. Thisenables dense wiring patterns with predictable space requirements fortrace routing programs. The close trace spacing in FIG. 6 is suitablefor low speed signals. Higher speed signals may require wider spacing toavoid cross-talk issues.

FIG. 7 is a schematic plan view of a circuit node 70 that connectsbetween an I/O pad 58 a on IC chip 71, and 110 pad 58 b on IC chip 72. Astacked contact 50 a is shown at I/O pad 58 a, as described in referenceto FIG. 5. Trace 73 is on a metal layer below the I/O pad layer (surfacelayer), for example on metal layer 7 in a circuit with 8 metal layers.Trace 73 contacts using two-level contact 33 a to trace 74, which is onmetal layer 8 in this example. A set of parallel metal traces on layer 8is shown, each trace having a width, w, of 5 microns or less in thepreferred embodiment. The separation, s, between traces is also 5microns or less in the preferred embodiment. The I/O pad pitch, P, canrange from 60-200 microns, and 95 microns is preferred. Circuit node 70continues from trace 74 to contact 33 b, contacting to trace 75 on metallayer 7 in the example, and terminates at I/O pad 58 b using stackedcontact 50 b.

Having explained the details of building a high density interconnectstructure in the form of a flexible multi-layer interconnection circuit,we shall now focus on assembly and testing of IC chips on theinterconnection circuit, to form a circuit assembly.

FIG. 8A shows circuit assembly 80 with multiple IC chips such as 81 and82, and other surface-mounted components such as 84 on multi-layerinterconnection circuit 20. Components 81-83 are preferably attached bythe flip chip assembly method, including stud bumps and wells filledwith solder as described herein. Alternatively, surface-mount componentsmay be attached using known solder re-flow techniques. Module accessport 84 provides an array of module access pads (I/O pads) forconnection to external signals and power, as well as for connection tointernal nodes of the interconnection circuit for testing purposes, aswill be further described. IC chip 85 is a special-purpose test chip inthe preferred embodiment. For testing different circuit types, such asdigital, analog, and radio frequency (RF), it may be desirable toassemble more than one special-purpose test chip. Alternatively, allforms of testing may be accomplished using external testers, accessedthrough module access port 84.

FIG. 8B represents a cross-sectional view of section XX of FIG. 8A.Circuit assembly 80 is supported on release layer 3 on top of glasscarrier 1. It includes interconnection circuit 20 plus attachedcomponents. IC chip 81 is attached using flip chip connections such as86, which will be further described with reference to FIG. 9.

FIG. 9A-9C shows a sequence of steps describing a first method of thecurrent invention for creating a flip chip bond (direct chipattachment). FIG. 9A shows that an IC chip 71 has been prepared forassembly by forming gold stud bumps such as 91 at 110 bonding pads suchas 92. Stud bumps 91 can be created using a Kulicke and Soffa 8098bonder, using the application of beat, pressure, and ultrasonic energy,as is known in the art. The process for forming the ball portion of thestud bump is the same as for a ball bonder (conventional wire bonder).If an 18-micron diameter gold wire is used, the bonder can be configuredto make stud bumps such as 91 with a ball diameter of 50 microns and anoverall height of 50 microns. The “beard” 93 is created by accuratelyshearing the gold wire, and according to Kulicke and Soffa, the tips ofthe beards can be coplanar within +2.5 microns across a 200 mm field..FIG. 9B shows 110 pads 58 as shaped for stacked contacts, as previouslydescribed in reference to FIG. 5A. Interconnection circuit 20 isattached to glass carrier 1 via release layer 3, and glass carrier 1 isproviding the necessary dimensional stability during assembly. Thedepression at a stacked contact may be used as a well for assemblypurposes. A metallization 95 is patterned over each I/O pad 58 as shown,to prevent diffusion of solder materials or dissolved gold intomulti-layer circuit 20, to provide an oxidation barrier, and also toprovide a solder-wetting surface. An acceptable sequence of layers formetallization 95 is an adhesion layer of aluminum, a solder diffusionlayer of nickel, and an oxide prevention layer plus solder wettablelayer of copper. This sequence is known in the art as under bumpmetallization, UBM. Multi-layer interconnection circuit 20 is furtherprepared for IC chip assembly by filling the wells with solder paste,thus creating a well filled with solder 94 at each I/O bonding pad 58.Solder paste 96 is applied using the wiping action of a squeegee overthe exposed surface. In the preferred embodiment, using a large glasspanel as the carrier, several million wells are typically created withone pass of the squeegee. Solder paste 96 is laterally confined by thewells. FIG. 9C shows a completed flip chip bond 86 of the currentinvention, with stud bump 91 inserted into well 94. Since the heightvariation of the stud bumps is held to approximately ±2.5 microns, andsince the beard is a ridge of small cross-section, and since gold is asoft and malleable material, a small amount of pressure applied to an ICchip will result in the tips of the beards conforming to and makinguniform contact with the bottoms of the wells. The depth of well 94 andthe softness of the solder paste provide a vertical compliance duringflip chip assembly, thus avoiding broken chips. In FIG. 9C solder paste96 has been melted and cooled to form solder 97, thus creating apermanent electrical and mechanical bond. Solder 97 forms a strongmechanical bond with the beard and the underside of the stud bump, aswell as a low resistance contact. Since the stacked contacts arepatterned photo lithographically they are accurately placed within a fewmicrons and their size and shape are repeatable, leading to high yieldassemblies. The pitch, P, is 95 microns in FIG. 9C, and is preferably inthe range of 60-200 microns. Solder paste 96 is preferably anindium-based material such as Indalloy 290 comprising 97% In and 3% Ag,with a melting point of 143° C. This material is designed to solder goldelectrodes with minimal dissolution of the gold, and without formingundesirable inter-metallic compounds that could impair the mechanicalintegrity of flip chip bond 86 or degrade the ability to perform rework.It also solidifies in a semi-rigid form, providing some additionalmechanical compliance compared with Pb:Sn solders. Additionally, it doesnot contain Pb which is an environmental contaminant. The melting pointof 143° C. is lower than that of all common production solders. This isgood for rework: less heating is required to melt the solder and asmaller overall temperature excursion results in less thermal stressduring each rework cycle.

The sequence of FIG. 9A-9C represents a cost-effective method ofcreating the wells filled with solder 94, because the well shapes arealready provided by the stacked contacts at each 110 pad. Thus aseparate process is not required to fabricate the wells. However, somemanufacturers may prefer not to use stacked contacts, and someinterconnection circuits may not include enough layers to createsufficiently deep wells. For these cases a second method is proposed forfabricating the wells, as shown in FIG. 10A-10D.

In FIG. 10A interconnection circuit 20 is shown, including flat I/O pads100, coated with metallization 95 as previously described. Again, glasscarrier 1 is present for dimensional stability. In FIG. 10B aplanarizing layer 101 of non-photo-definable polymer such as BCB isformed on top of interconnection circuit 20, at a thickness ofapproximately 15 microns when cured. Polymer layer 101 is masked andetched using known dry etching techniques to create openings (wells) 102above the pads. The diameter of the wells is approximately 34 microns inthe preferred embodiment. The openings preferably have vertical walls asshown, providing maximum resistance to shear forces in the finalassembly (with chips attached). Vertical walls are not generallyproducible using photo-defined polymers such as photo BCB, and this iswhy dry etching is used, preferably with an anisotropic etchingcharacteristic as is known in the art. Polymer layer 101 forms thesolder paste mask and is typically not removed, i.e., it remains a partof the finished circuit assembly. This special assembly layer isnumbered 102. Solder paste 96 is wiped over the exposed surface using asqueegee and is laterally confined by openings 103. By this means, asecond preferred embodiment of wells filled with solder 104 is createdas shown in FIG. 10C, and a second embodiment of bonded connection 105including a gold stud bump 91 mated with a well filled with solder, asshown in FIG. 10D. A strong mechanical bond is formed between gold studbump 91 and I/O pad 100 because solder 97 adheres strongly to stud bump91 and to metallization 95 (which in turn is strongly bonded to I/O pad100), and solder 97 is well contained and structurally supported byopenings 103. There are no fragile leads exposed to the assemblyprocess, because fragile signal traces (not shown) are terminated in I/Opads 100 such that the traces are a short distance away from flip chipattachment 105; this means that a direct attached component can bereworked (replaced) without danger of damaging the signal traces. 110pad pitch, P, is shown at 95 microns, enabled by the photolithographicprecision employed for patterning the various layers, plus the smalllateral dimension of gold stud bumps (approximately 50 microns for thestud bumps shown), and the dimensional stability of the underlyingcarrier. A range of I/O pad pitch from 60-200 microns is preferred.

It is a primary goal of the current invention to provide flip chipassemblies that can be reworked when a defective chip needs replacement.The process should be simple and effective, and robust enough to performas many times as may be necessary. Consequently, it is important tochoose a solder composition that does not significantly dissolve thestud bump material, does not form brittle intermetallic compounds, andfor which the melting point is not significantly increased duringassembly or rework cycles. Gold is the preferred material for the studbumps. As previously described, an example of a solder material that iscompatible with gold stud bumps and the flip chip bonding methodsproposed herein is indium-based solder Indalloy #290, obtainable fromIndium Corporation of America, Utica, N.Y. The amount of paste in a wellis approximately 1.5×10-8 gm in the second preferred embodiment, at amaterial cost of approximately US$3.48 per gram. For either method offabricating the wells, the cost of filling the wells is low when largeglass carriers are used because typically several million wells arefilled in parallel. Preferably, a squeegee is used to wipe solder pasteover the mask (or over the I/O pad indentations provided at stackedcontacts in the first embodiment) to deposit the paste in the wells. Thecost per stud bump using a Kulicke and Soffa 8098 bonder isapproximately 0.03 cents, based on data provided by Kulicke and Soffafor 200,000 bumps on an 8-inch wafer. Consequently, the cost of eachflip chip connection in the current invention, including the stud bumpand the corresponding well, is estimated at less than 0.05 cents orUS$0.0005. This compares with current costs per connection ofapproximately 0.3 cents for wire bonds in a comparably similar circuitassembly, and typically higher costs for other flip chip attachmentmethods. Such a low cost for the current invention may mean thatadditional test points can be included within a satisfactory overalltest cost.

The rework process for replacing a defective IC chip on a circuitassembly of the current invention is as follows. Heat is applied using ahotplate under glass carrier 1, typically bringing the carrier to atemperature below the solder melting point. For the preferred solder,Indalloy 290, this temperature may be approximately 118° C. Hot inertgas is applied to the backside of the chip; the gas is directed andconfined so as to melt the solder of a single component, avoidingneighboring components. When the solder melts at around 143° C., thedefective component is removed by withdrawing the stud bumps from thewells. The surface around the affected wells is inspected. Touchup mayinclude cleaning of the surface area around the wells and addition ofmore solder paste into the wells; the board is then ready for areplacement component.

The bump/well connection method described in FIG. 9 and FIG. 10 can beapplied to fabricating a connection between any pair of electroniccomponents. It only requires that bumps be formed on one part (the malepart) and corresponding wells be formed on the other part (the femalepart). Accordingly, IC chips and other components may be provided witheither wells or bumps, as long as the matching structure is fabricatedon the mating part.

FIG. 11A represents an example of a module access port 84 withindividual module access pads such as 110 arrayed as shown. The moduleaccess pads provide a means for electrical connection frominterconnection circuit 20 or from circuit assembly 80 to otherelectronic assemblies or devices, and include provision for datasignals, control signals, and power. As will become apparent, thebump/well bonding structures described for these pads are similar oridentical to those described for the pads of IC chips.

An external tester that is connected to assembly 80 through moduleaccess port 84 may be used to validate the integrity of theinterconnection circuits prior to assembling IC chips and othercomponents. It may be advantageous to provide a module access pad forevery node on the interconnection circuit, to provide 100% testcoverage. If a module cable of the current invention is used, as furtherdescribed with reference to FIG. 15 through FIG. 19, module access pads110 can have a pitch of less than 100 microns, just like flip chip bonds86 and 105. If an interconnection circuit has 50,000 nodes correspondingto a medium-complexity system board, then the area occupied by themodule access port is only 5 square centimeters at a bonding pitch of100 microns (2.24 cm on a side). Testing for short circuits is typicallyprovided between all of the nodes, and testing for open circuits mayalso be performed on critical nets (distributed nodes). This is similarin concept to a “bed-of-nails” test that is typically performed onconventional printed circuit boards. In some cases, repair of defectiveinterconnection circuits may be appropriate, perhaps using focused ionbeams, FIB, for cutting traces, or other types of beams for building newones.

It may be advantageous to apply the bump/well assembly method to theproblem of aligning a pair of components. In this case, stud bumps areprovided on a face of one of the components, and a matching set of wellsis provided on a face of the other component. A two-step alignmentprocedure is preferred for precise alignment. In the first step,alignment within a few microns is achieved by the inherent precision oflocating the stud bumps in the wells. In the second step, the alignmentmay be fine-tuned to within a fraction of a micron displacement errorand small angular errors by monitoring an alignment-sensitive parameterof the combined pair, and optimizing the alignment using this parameterwhile the solder is molten. This procedure could be used to alignoptical assemblies for example, and signal to noise ratio of a lightbeam traversing the combined assembly may be a suitable parameter tomonitor while optimizing the alignment. The bumps and wells could belocated sufficiently far from the light path so as not to interfere. Itmay also be desirable to form the wells in a hard material, for greaterrigidity and precision.

A circuit assembly such as 80 will typically require several differentworking voltages for operation. Preferably, power at the highest workingvoltage will be delivered through module access port 84 (preferablyusing multiple pins, in a distributed fashion), and local converters andregulators that are implemented on one or more IC chips will provideother working voltages as required. The converters and regulators may beprogrammable in order to adjust the working voltages for testingpurposes.

The layout of circuit assembly 80 is so dense, both in the fine linetraces and in the fine pitch assembly, that conventional connectors andcables are not well suited for interconnecting modules of this type;they would occupy a large fraction of the total module space.Consequently, part of the current invention is to provide an effectivemeans for connecting circuit assemblies to testers, and circuit modulesto other circuit modules or to other electronic systems employingdifferent manufacturing methods. It is proposed that the same methodsused to fabricate circuit assembly 80 can also be used to fabricate testfixtures and module cables.

In FIG. 11B, a test fixture 111 is shown connecting to module accessport 84 of circuit assembly 80. A redistribution of the module accesspads 112 is provided on glass substrate 113, so as to connectconveniently to an external device using a low cost cable 114 havingrelatively large features. For small arrays of module access pads, asingle layer of aluminum may be patterned on glass substrate 113, with aone-to-one connection between module access pads in the module accessport, and corresponding pads in the redistributed array. This can beaccomplished (with no signal crossovers) if redistributed array 112 is ascaled mirror image of the module access pad array. For larger arrays, amulti-level interconnection circuit may be required. As previouslydescribed in reference to FIG. 9 for forming stud bumps on IC chips, sostud bumps may be formed at I/O pads on test fixture 111. They may beprovided at a pad pitch of 100 microns or less at the small endcontaining the module access port, to mate with wells filled with solderat each of the module access pads. A conventional flexible circuit maybe employed to connect an external device (such as a tester) to pads inredistributed array 112. Such a flex circuit may have copper conductors,and may include bumps that connect by contact pressure with pads ofredistributed array 112. These pads may be enlarged for ease ofalignment and robustness with respect to unmatched CTEs. They may alsobe plated with gold for a low-resistance contact. A primary purpose oftest fixture 111 is to provide connection means for verifying theintegrity of the multi-layer interconnection circuits before anycomponents are assembled. Secondary purposes may include testing of thecircuit assembly 80. If test chips are provided on the module, then testfixture 111 may be used to connect these chips to an external tester forverification. Connections to the module access port can be unmade in thesame way that defective IC chips can be reworked, by heating the solderand withdrawing the stud bumps.

Heat is applied using a hotplate under the glass carrier supportingcircuit assembly 80 and hot inert gas is applied to a localized regionon the topside of glass substrate 113. The hot gas is directed andconfined so as to melt the solder associated with the targetedconnector, and not the solder of neighboring components. Afterwithdrawing the stud bumps from the wells the surface is inspected.Touchup may include cleaning of the area around the wells and additionof more solder paste to the wells. Then another connection to the moduleaccess port can be made for the same or a different purpose.

Before permanently assembling a module cable to the circuit assembly, itis desirable to coat the top surface with a dielectric layer and a metallayer, as part of the process to create a hermetic or semi-hermeticmodule. The dielectric layer prevents shorting of components when themetal layer is subsequently applied. The metal layer provides a shieldat the top surface, except for small holes at the module access pads, aswill be further described in reference to FIG. 13. FIG. 12 shows across-sectional view like FIG. 8B except that some module level coatingshave now been applied to evolve circuit assembly 80 into circuitassembly 120. At the edges of components, where vertical faces meetinterconnection circuit 20, fillets 121 are shown. They provide supportfor dielectric coating 122 which is a thin passivating layer of Parylenein the preferred embodiment. The material of fillet 121 is siliconerubber or other inert material. It is applied by extruding a bead of thematerial, then following with a narrow spatula to shape the bead into atriangular cross-section. The fillet material is then cured, anddielectric film 122 and metal film 123 applied. Metal film 123 is thefirst topside module-level metal film. It provides a conductive coatingin the region of the module access port, except for small openings atthe module access pads. This area will not be covered by secondmodule-level topside metal, because when that coating is deposited amodule cable will be connected at the module access port and will blockthe deposition path, as will be further described in reference to FIG.18A. Metal film 123 is preferably aluminum with a thickness ofapproximately one micron. Other metals and thicknesses can be used. Onemicron of aluminum provides good hermetic protection plus someelectromagnetic shielding, at reasonable cost. Depending on the severityof the electromagnetic environment, it may be desirable to substantiallyincrease the thickness for effective shielding performance.

FIG. 13A-FIG. 13C show how the first topside module-level metal film ispatterned near the module access pads of circuit module 120; they alsoshow the similarity between flip chip connections 86 and 105 alreadydescribed for attaching IC chips and module access port connections suchas 131, shown in FIG. 13C. FIG. 13A shows multi-layer interconnectioncircuit 20 on release layer 3 on glass carrier 1. A flat 110 bonding pad100 is shown, which in this case is a module access pad of module accessport 84. Alternatively, if stacked contacts are provided at each I/O padof module access port 84, then the indentations already provided by thestacked contacts may be employed for the wells, as described inreference to FIG. 9. Metallization 95 is shown over bonding pad 100, aspreviously described. The pitch between wells filled with solder 104 is95 microns in the figure, with a preferred range of 60-200 microns.Module-level packaging layers 122 and 123 are also shown. Layer 122 is apassivating layer of Parylene, and layer 123 is a hermetic and shieldinglayer of metal as previously described. Opening 103 is patterned andfilled with solder paste 96 to form well 104 as previously described.FIG. 13C shows an enlarged portion of a module cable 132 inverted overthe wells; it is attached to glass carrier 136 for dimensional stabilityduring assembly. Gold stud bumps 91 are bonded to metal pads 133 thatsit on interconnection circuit 134 on top of release layer 135, on topof glass carrier 136 (inverted in the figure). Referring to the shapeand size of multi-layer circuit 22 in FIG. 2, and understanding that acommon manufacturing process maybe used to fabricate circuit assemblieslike 80 and module access cables like 132, it can be seen that 132, 135,and 136, may actually be the same as 22, 3, and 1, respectively. Thepath 137 for water to migrate into interconnection circuit 20 is shown.The surfaces surrounding the wells are coated with dielectric 122 andmetal layer 123 to provide a barrier to water at the module access pads,in the area surrounding but not including the wells. The solder in thewells also provides an effective water barrier, leaving just a narrowentry point for water, and a long path 137 through polymer layer 101 inFIG. 13C. Polymer layer 101 provides a weak barrier to water. Thus themodule access pads are semi-hermetic. Since the area of the moduleaccess port is typically small compared with the total surface area ofthe module, the total exposure to water is limited to a semi-hermeticportion of small extent.

Referring back to FIG. 8A, the assembly and test sequence of theelectrical components such as IC chip 81 will now be discussed. Beforeassembling any components, a test fixture such as 111 is temporarilyattached to module access port 84, with the other end connected to atester. Multi-layer interconnection circuit 20 is tested for opens andshorts, and rejected if defective. The first chip to be assembled may betest chip 85. This chip is itself verified by an external tester withcapabilities suitable for testing both its functional and parametricspecifications. If defective, test chip 85 is replaced using the reworksequence previously described. Once installed and verified, the testchip is capable of testing the remaining components at circuit speed.Alternatively, a completed flip chip assembly like 80 may be testedusing an external tester, connected using test fixture 111. If desired,testing can be performed at an elevated temperature applied to theentire circuit assembly, using a heater wider the glass carrier. Afterthe circuit assembly has been completely assembled and tested, the testassembly is removed so as not to obstruct the coating of an additionaltop surface metal layer, as further described in reference to FIG. 18A.The overall sequence for creating a tested circuit assembly is presentedas a flow graph in FIG. 20.

FIG. 14 shows a conventional RF sputtering chamber 140, used for vacuumdeposition of metal layers. Vacuum chamber 141 has an inlet port for asputtering gas such as argon, and an exit port connected to a vacuumpump as shown. Chamber 141 includes a top electrode 142 that isconnected to an RF source (not shown), and a counter electrode 143. Thepart to be coated 144 normally sits directly on counter-electrode 143.During sputtering, a plasma 145 of ionized gas is formed between the topelectrode and the counter-electrode as shown. These details are providedas background for a modification to this apparatus, further described inreference to FIG. 18A and 18B, wherein the counter-electrode is modifiedto include a pedestal.

FIG. 15 shows module cable 132 of the present invention, in plan view.It is connected to circuit module 120, before glass carrier 1 is removedto apply the bottom side metal.

FIG. 16 shows module cable 132 with an array of module access pads ateach end of the bottom surface (made visible in the figure fordescriptive purposes). On the top surface, a scribe line 161 has beenscored in the glass surface with a scribing tool, marking the placewhere the glass carrier will later be cracked into two separate pieces.Alternatively, scribe line 161 may be a shallow cut with a diamond saw,positively defining the location of the break to be made, while leavingenough glass thickness to provide adequate strength for handling. FIG.17A through FIG. 17C shows the sequence for connecting one end of modulecable 132 to circuit assembly 120. To maintain the necessary dimensionalstability, it is critical that a glass carrier be present on both sidesof the connection interface while the bond sites on both sides arealigned and the bonds are permanently created. In FIG. 17A, module cable132 includes glass carrier 1 with release layer 3 as previouslydescribed, and a single or multi-layer interconnection circuit 20 withstud bumps 91 attached at each bonding pad of an array of module accesspads. A similar array of stud bumps 172 is shown at the other end ofmodule cable 132. Interconnection circuit 20 connects between I/O padsof the two arrays in a one-to-one relationship. A cover or sheath 173protects the unused end of module cable 132, including the second arrayof gold stud bumps 172, until module 120 is connected to another circuitmodule, or to another electronic system. Module cable 132 can beenvisaged in a more complex form, including three or more arrays ofmodule access pads, with each array connecting to a circuit module, orto an electronic system other than a circuit module. One or more of thearrays of module access pads may be redistributed with a larger padpitch for greater ease of connection to a particular piece of electronicequipment, as was described for test fixture 111. As previouslydescribed, when correct alignment has been achieved heat is applied toform permanent bonds between stud bumps on module cable 132 andcorresponding wells filled with solder on circuit assembly 120; FIG. 17Ashows module cable 132 attached to circuit assembly 120. This studbump/well connection is preferably as described in FIG. 13C. FIG. 17Bshows that a bending force has been applied, to crack the glass carrierinto two pieces at scribe line 161. Crack 174 is shown. In FIG. 17C, oneportion of glass carrier 1 has been removed by peeling the glass pieceaway from interconnection circuit 20, leaving behind interconnectioncircuit 20 attached to circuit assembly 120 at each of the module accesspads. A module cable that is connected at one end is labeled 175.

It is desired to create a continuous metal envelope around the circuitassembly including the interconnection circuit and the attachedcomponents, with only a small opening at each of the module access pads.For hermeticity and effective electromagnetic shielding it is criticalthat the top and bottom metal layers form an overlapping seam ofcontinuous and void-free metal at the edge of the circuit assembly. Apreferred method for achieving this is shown in FIG. 18A-1813. FIG. 18Ashows counter-electrode 143 previously described in reference to FIG.14; for convenience the other parts of RF sputtering chamber 140 areomitted in FIG. 18A-18B. Normally, a part to be coated in a sputteringchamber requires deposition on the top surface only and is placeddirectly on the counter electrode. In the current invention, counterelectrode 143 is modified to include a pedestal 181 that provides an airgap of a few millimeters between circuit assembly 182 and the opposingsurface of the counter electrode. The effect of this arrangement is thatthe second top layer metal film 183 will also coat around the edge ofthe circuit assembly with a thickness that tapers to zero, 184,underneath circuit assembly 182. Circuit assembly 182 corresponds tosection YY of FIG. 8A. The preferred metal film 183 is aluminum, with athickness of one micron, that adds to the first module-level metal film123, also with a thickness of one micron. Module cable 1 75 is folded tocreate a minimum footprint blocking the path of deposited metal tocircuit assembly 182. After the second top layer metal film 183 has beendeposited, the circuit assembly is flipped on pedestal 181 as shown inFIG. 18B. A cutout 185 is provided in pedestal 181 to store module cable175, in a manner that leaves the edges of circuit module 182 exposed forcoating. After deposition of bottom side metal 186, circuit assembly 182becomes circuit module 187, with the name “module” implying a completeenvelope of conductive material. Again, the coating extends around theedges, 188. The net result is a continuous metal film covering thetopside, the bottom side, and the edges, to form a complete envelopearound the circuit assembly. An alternative method for creating a metallayer with a coating that extends around the edge like 188 is to use avacuum evaporator. In this case the circuit assembly is held on acarousel, and the carousel executes a planetary motion with respect tothe evaporation source to effect coating of a substrate from manydifferent angles, as is known in the art.

FIG. 19 shows system module 187 of the current invention afterwithdrawal from the sputtering chamber, with all processing completed.The interconnection circuit and attached components of module 187 havebeen thoroughly tested as an integrated system. Module 187 includes aconductive envelope 190 comprised of multiple overlapping coatings.Conductive envelope 190 continuously covers the module except for smallopenings at the module access pads. Module cable 175 is attached usingfine pitch connections. Conductive envelope 190 provides a hermeticseal. It also provides an effective electrical screen for reducingelectromagnetic interference between the module and other electroniccomponents. It reduces electromagnetic radiation, EMR, produced by themodule and also reduces electromagnetic susceptibility, EMS, by reducingthe effect of external electromagnetic waves on circuits within themodule. With its carrier removed module 187 is flexible except that itmay be constrained by assembled components that are rigid.

FIG. 20A-20C show various cables of the current invention. FIG. 20A is asimple cable comprising an interconnection circuit having multipletraces 201 connected at one end to port 202 and at the other end to port203. Interconnection circuit 201 may have a single layer of conductors,or multiple layers as described in reference to FIG. 3 and 4. Port 202may be an input port comprising all of the input signals to the cable,and port 203 may be an output port comprising a matching set of outputsignals from the cable. Signals may also be bidirectional in someapplications, so that a port may consist of mixed input and outputsignals. Also the ports can be implemented with either gender on thecable side; all that is required is that there are bumps on one side ofthe connection and wells on the other side, and this is generally truefor all of the cables of the current invention. Within a single cable,some ports may consist of bumps and other ports of wells. The wells canbe fabricated as described in reference to FIG. 10C, for example. FIG.20B shows a cable 204 wherein port 205 has been expanded using aredistribution array similar to the redistribution array described inreference to FIG. 11B, to allow greater spacing between the I/O pads.This may simplify the task of attaching to an external component orsystem having a different set of pad spacing requirements. FIG. 20Cshows a cable 206 with multiple fingers 207, each finger having one ormore ports.

FIG. 21 shows a summary in the form of a flow chart of theaforementioned process for constructing a circuit assembly of thepresent invention. If adequate depressions are formed by making stackedcontacts at each I/O pad, then a separate step for fabricating thesolder wells may not be required.

FIG. 22 shows a summary in the form of a flow chart of theaforementioned process for constructing a system module from a circuitassembly, where the module includes a module cable (if required) and asurrounding envelope of conducting material.

FIG. 23 shows an alternative embodiment of a circuit assembly, such asfor a blade server component, 230, of the current invention. A singlelarge interconnection circuit 231 contrasts with the array of smallerinterconnection circuits 20 shown in FIG. 2. Processing groups of ICchips 232 are arrayed as shown, including individual IC chips such as233. Each processing group may include processor chips, memory chips,and bus-interface chips, for example. The dense interconnection circuitsand assembly methods of the current invention provide for a largecomputational capability on circuit assembly 230. A module access port234 similar to 84 is shown, for connecting to other systems, includingtest systems. A group of special-purpose programmable test chips isprovided, 235, and also a maintenance group of IC chips 236, formaintenance of the blade server. For example, any failure occurring in aprocessing group may be automatically detected by a background processrunning on maintenance group 236, which will reconfigure blade servercomponent 230 to bypass the defective group for any further operations.

In FIG. 24, circuit assembly 230 has been converted into circuit module240 by application of module-level coatings, as previously described.FIG. 24 shows an edge fragment of circuit module 230 coupled with a heatsink 241. Circuit module 240 includes multi-layer interconnectioncircuit 242 which may differ from interconnection circuit 20 because itis configured for high-speed operation using differential signal pairs,for example. IC chips such as 243 are preferably attached using flipchip bonds such as 105, previously described. Similarly, module-levelcoatings including dielectric layer 191 and conductive envelope 192 arefabricated using the same methods as previously described for circuitmodule 190. Circuit module 240 is attached to heat sink 241 at the facesof chips such as 233, using a layer of thermal material 243 at eachattached IC chip. Material 243 may be thermal grease or a conductiveepoxy, and may be applied by printing as a thick film onto the topsurface of module 240. Heat sink 241 preferably contains are-circulating coolant fluid, to extract heat effectively using a heatsink of small volume. Circuit module 240 and heat sink 241 may becombined to form a blade server component that can be inserted into aserver chassis, with appropriate provisions for electrical and plumbingconnections. The thermal path from active transistor junctions on ICchips such as 233 to heat sink 241 has low thermal impedance. Itincludes only one layer of non-conducting material, which is preferablya thin layer of Parylene 191 in the current invention. This means thatIC chips of blade server module 240 can operate at high power levels,without exceeding their maximum junction temperature specifications.

1. A method for fabricating an electronic circuit comprising the stepsof: providing a rigid carrier; applying a base dielectric layer on saidrigid carrier; fabricating one or more interconnection circuits havingexposed input/output pads on said base layer; fabricating wells at saidinput/output pads of said interconnection circuits; and filling saidwells with conductive material.
 2. The method of claim 1 and includingthe additional step of attaching electronic components to saidinterconnection circuits to form an electronic assembly, wherein each ofsaid components has a conductive bump at each of its input/output pads,and each of said conductive bumps is inserted into said conductivematerial in one of said wells.
 3. The method of claims 1 including thestep of fabricating a release layer interposed between said rigidcarrier and said base dielectric layer.
 4. The method of claim 2including the step of fabricating a release layer interposed betweensaid rigid carrier and said base dielectric layer.
 5. The method ofclaim 3 including the step of separating said interconnection circuit orelectronic assembly from said rigid carrier at said release layer, aftercompleting said circuit or assembly.
 6. The method of claim 1 whereinsaid interconnection circuits comprise multiple dielectric andconducting layers.
 7. The method of claim 2 wherein said interconnectioncircuits comprise multiple dielectric and conducting layers.
 8. Themethod of claim 3 wherein said interconnection circuits comprisemultiple dielectric and conducting layers.
 9. The method of claim 4wherein said interconnection circuits comprise multiple dielectric andconducting layers.
 10. The method of claim 4 wherein said release layeris not present near the edges of said rigid carrier, to provide an edgeregion of strong adhesion between said base dielectric layer and saidrigid carrier.
 11. The method of claim 10 wherein said release layer isadditionally not present in streets at the periphery of a plurality ofsaid interconnection circuits.
 12. The method of claim 1 wherein saidwells are filled with said conductive material using a squeegee.
 13. Atrace routing method for a multi-layer interconnection circuitcomprising the steps of: providing stacked contacts with trace stubs atinput/output pads of said interconnection circuit; and limiting contactsbetween conductive layers to two-level contacts in routing areas wheremaximum routing density is desired.
 14. A method for forming wellsfilled with conductive material on an interconnect circuit comprisingthe steps of: applying a layer of dielectric material on top of saidinterconnect circuit; forming openings in said dielectric layer at eachinput/output pad of said interconnect circuit; and depositing saidconductive material in said openings to form wells filled withconductive material.
 15. The method of claim 14 wherein said conductivematerial is deposited in said openings using a squeegee.
 16. A methodfor assembling an integrated circuit chip having input/output pads on aninterconnection circuit having bonding sites corresponding to saidinput/output pads comprising the steps of: providing a conductive bumpat each of said input/output pads; fabricating wells filled with solderat each of said bonding sites; inserting said conductive bumps into saidwells filled with solder; and heating and cooling said solder to makepermanent connections.